The processes and apparatus of the present invention concern melt spinning fluoropolymers into single filaments or multi-filament yams at high spinning speeds.
Melt spinning of thermoplastic copolymers based on tetrafluoroethylene is known. However, there is considerable economic incentive to drive fiber spinning rates ever higher for these high value polymers. One problem facing processes of melt spinning is that at high shear rates, melt fracture occurs which becomes evident as surface roughness in the extruded fibers. Since the critical shear rate for the onset of melt fracture decreases with increasing melt viscosity, ways to decrease melt viscosity have centered on raising the temperature of the melt. However, in many polymers including thermoplastic copolymers based on tetrafluoroethylene, the polymer exhibits thermal degradation before any significant decrease in melt viscosity can be achieved.
Fibers of polytetrafluoroethylene (PTFE) homopolymer are also highly valued, particularly for their chemical and mechanical properties, such as low coefficient of friction, thermal stability and chemical inertness. However, processing by melt spinning has proved elusive. Since polytetrafluoroethylene homopolymer fibers are conventionally formed by a dispersion spinning process involving many steps and complicated equipment, there is great economic incentive to find a method for melt spinning such fibers.
The problem of spinning fibers from high viscosity polymer melts has been previously addressed for polyesters. In U.S. Pat. No. 3,437,725 a spinneret assembly is described having a top plate, a heating plate and a lower plate with a spacer providing air space between the top plate and the heating plate. Hollow inserts, one for each filament to be spun, are placed in the top plate and extend to the bottom face of the lower plate. Molten polymer is fed into the inserts for spinning through capillaries. An electrical heater supplies heat to maintain the lower plate, heating plate and lower portions of the inserts at a temperature at least 60xc2x0 C. higher than the temperature of the supplied molten polymer. Heated capillary temperatures ranging between 290 and 430xc2x0 C. were listed in examples for spinning polyesters. No mention is made of any fluoropolymer or temperatures needed to melt spin fluoropolymers at high spinning speeds.
The present invention provides a process for melt spinning a composition comprising a highly fluorinated thermoplastic polymer or a blend of such polymers, comprising the steps of melting a composition comprising a highly fluorinated thermoplastic polymer or a blend of such polymers to form a molten fluoropolymer composition; conveying said molten fluoropolymer composition under pressure to an extrusion die of an apparatus for melt spinning; and extruding the molten fluoropolymer composition through the extrusion die to form molten filaments, said die being at a temperature of at least 450xc2x0 C., at a shear rate of at least 100 secxe2x88x921, and at a spinning speed of at least 500 m/min.
The present invention also provides a process for melt spinning a composition comprising polytetrafluoroethylene homopolymer, comprising the steps of melting a composition comprising a polytetrafluoroethylene homopolymer to form a molten polytetrafluoroethylene composition; conveying said molten polytetrafluoroethylene composition under pressure to an extrusion die of an apparatus for melt spinning; and extruding the molten polytetrafluoroethylene composition through the extrusion die to form molten filaments.
The present invention further provides an apparatus for melt-spinning fibers comprising a spinneret assembly comprising means for filtering; a spinneret; an elongated transfer line, said transfer line being disposed between said filtration means and said spinneret; means for heating said elongated transfer line; means for heating said spinneret; and an elongated annealer disposed beneath said spinneret assembly.
With respect to the process for melt spinning highly fluorinated hermoplastic polymer at an extrusion die temperature of at least 450xc2x0 C., his high minimum temperature is required for the perfluorinated luoropolymers. Lower extrusion die temperatures can be used for hydrogen-containing highly fluorinated thermoplastic fluoropolymers, such as ethylene/tetrafluoroethylene copolymer (ETFE), which have lower melting points than the perfluorinated fluoropolymers, such as in the range of 250-270xc2x0 C. for ETFE. These fluoropolymers can be spun into yarn in accordance with the process of the present invention at extrusion die temperatures which while less than 450xc2x0 C., are still substantially greater than the melting point of the polymer. Thus, one embodiment for the process for melt spinning a composition comprising highly fluorinated thermoplastic polymer (including a blend of such polymers) comprises melt spinning at least one filament at a temperature of at least 90xc2x0 C. greater than the melting point of said polymer. Such melt spinning temperature is the same as the extrusion die temperature mentioned above. Preferably such melt spinning temperature is at least 340xc2x0 C., while for the perfluorinated thermoplastic polymers, the minimum melt spinning temperature remains at 450xc2x0 C.
Another process for melt spinning highly fluorinated thermoplastic polymer, comprises carrying out the melt spinning into at least one filament and shielding the resultant molten filament from turbulent air to delay solidification of the filament until it reaches a distance of at least 50xc3x97 the diameter of the die through which the filament is melt spun.
While each of the foregoing described processes can be carried out on the melt spinning of one filament of the fluoropolymer, it is preferred that the melt spinning produce a plurality of filaments, preferably at least about 3, more preferably at least about 10, to form a yam thereof.
Another embodiment of the present invention is the melt spun yam itself. It has been found that in the melt spinning of the highly fluorinated thermoplastic polymers in accordance with the process of the present invention, at least about 90xc2x0 C. above the melting point of the polymer in general and at a temperature of at least about 450xc2x0 C. for the perfluorinated thermoplastic polymers,.or utilizing the shielding of the molten polymer to uniformly cool the filament(s) and thereby delay solidification, the resultant yam, whether monofilamentary or multifilamentary, has a novel cross-sectional structure, characterized by the core of the filament(s) having a greater axial orientation than the surface of the filament(s). In the normal melt spinning of such polymers, i.e. at temperatures considerably below those used in the present invention for the respective polymers being melt spun into filament(s), orientation of the molecules within the filament occurs upon the drawing of the yarn, either at a high rate of melt draw from the spinneret or such melt stretch followed by draw of the yam after it has solidified, i.e. draw below the melting point of the copolymer. Normally, such stretch, whether melt stretch or melt stretch plus subsequent draw causes the highest orientation of the molecules making up the filament to occur at the surface of the filament, because that is where the shear stress on the copolymer is the greatest, by virtue of the filament cooling from the surface of the filament before the core cools. Thus, while the molecules at the surface of the filament become aligned in the axial direction of the filament, the molecules in the core of the filament show less alignment. Draw of the filament accentuates the difference between surface and core orientations. This orientation phenomenon is further described in A. Ziabicki and H. Kawai, High-Speed Fiber Spinning, John Wiley and Son (1985) on p. 57. Filament(s) present in the highly fluorinated thermoplastic polymer yam of the present invention have reverse orientation, wherein the molecular orientation is greater in the core than at the surface of filament(s) present in the yam.
Drawing of the yarn after melt spinning can produce a variation on the above-described novel structure, namely wherein the orientation at the surface of the filament is no greater than the orientation at the core of the filament. Thus the orientation present at the surface of the filament can be the same as the orientation present in the core of the filament. The orientation difference between surface and core diminishes from that described above with increasing draw ratio. Thus, as the draw ratio reaches at least about 3, the detection of lesser orientation at the surface becomes more and more difficult.
In terms of forming the novel yarn of the present invention, the process of the present invention can also be described as melt spinning the polymer at a temperature above the melting point of the polymer which is effective to produce such yarn wherein the orientation in the filament(s) thereof is either greater in the core of the filament than at the surface thereof or the orientation at the surface of the filament is no greater than in the core thereof. The parameters of minimum shear rate and spinning speed described above are preferred for each of the process definitions for the present invention.
The present invention is particularly noteworthy in producing yam of ethylene/tetrafluoroethylene copolymer of high tenacity and at high rates and of fine denier/filament sizes and high denier uniformity along the length of the yarn, a preferred embodiment being set forth in Example 34. Preferred ETFE yams have a tenacity of at least 3.0 g/den and tensile quality of at least 8. Even more preferred ETFE yams are those having a tenacity of at least 3.0 g/den and an X-ray orientation angle of less that 19xc2x0. Each of these preferred yams, more preferably have a tenacity of at least 3.2 g/den, and the ETFE from which the yam is made has a melt flow rate of less than 45 g/10 min. These yarns while preferably having the orientation within filaments as described above are not limited to yams having such orientation.
The availability of the ETFE yam just described has enabled such yam to be used in a wide variety of applications, as disclosed in Examples 27 to 33.